Unseating Naturalism
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Unseating Naturalism

Recent Insights from Developmental Biology

arwin considered some of the best evidence for his theory to be the striking resemblance of vertebrate embryos at an early stage of their development. He wrote in The Origin of Species that “the embryos of mammals, birds, fishes, and reptiles” are “closely similar, but become, when fully developed, widely dissimilar.” He argued that the best explanation for their embryonic similarity was that such animals “are the modified descendants of some ancient progenitor.” According to Darwin, “the embryonic or larval stages show us, more or less completely, the condition of the progenitor of the whole group in its adult state.” (Darwin, 1859, pp. 338, 345)

Darwin believed that evolutionary changes tend to occur in the later stages of development and are gradually pushed back into embryogenesis, with the result that embryonic development bears the imprint of past evolution (in Ernst Haeckel’s words, “ontogeny recapitulates phylogeny”). The doctrine of recapitulation fits so nicely with Darwin’s theory that it has endured to the present, and can be found in many modern biology textbooks. But it was clear to embryologists even during Darwin’s lifetime that it did not fit the facts. Nineteenth-century embryologist Karl Ernst von Baer pointed out that although vertebrate embryos resemble each other at one point in their development, they never resemble the adult of any species, present or past. The most that can be said is that embryos in the same major group (such as the vertebrates, which include fishes, reptiles, birds, and mammals) tend to resemble each other at a certain stage before they develop the distinguishing characteristics of their class, genus and species. (Gould, 1977; Hall, 1992; Raff, 1996)

Darwin and his followers ignored these difficulties, however, and the modern synthesis excluded embryology entirely. Only in the past twenty years, with the rise of developmental genetics, has comparative embryology attracted significant interest from evolutionary biologists. One result of this renewed interest has been the recognition that patterns of early development do not fit the Procrustean bed of recapitulationism.

Although it is true that vertebrate embryos are somewhat similar at one stage of their development, at earlier stages they are radically dissimilar. After fertilization, animal embryos first undergo a process called “cleavage,” in which the fertilized egg divides into hundreds or thousands of separate cells. During cleavage, embryos acquire their major body axes (e.g., anterior-posterior, or head-to-tail, and dorsal-ventral, or back-to-front). Each major group of animals follows a distinctive cleavage pattern; among vertebrates, for example, mammals, birds, fishes, and reptiles cleave very differently. (Gilbert, 1994)

Animal embryos then enter the “gastrulation” stage, during which their cells move relative to each other, rearranging themselves to generate basic tissue types and establish the general layout of the animal’s body. The consequences of this process are so significant that embryologist Lewis Wolpert has written that “it is not birth, marriage, or death, but gastrulation which is truly the important event in your life.” (Wolpert, 1991, p. 12) Like cleavage patterns, gastrulation patterns vary markedly among the major groups of animals, including the different classes of vertebrates. (Elinson, 1987)

Only after gastrulation do the embryos of mammals, birds, fishes, and reptiles begin to resemble each other. In the “pharyngula” stage, every vertebrate embryo looks vaguely like a tiny fish, with a prominent head and a long tail. The neck region of a vertebrate pharyngula also has a series of “pharyngeal pouches,” or tiny ridges, which recapitulationists misleadingly refer to as “gill slits.” Although in fish embryos these actually go on to form gills, in other vertebrates they develop into various other head structures such as the inner ear and parathyroid gland (Lehman, 1987) The embryos of mammals, birds and reptiles never possess gills.

Therefore, Darwin’s belief in recapitulation is belied by the evidence. Embryologists have occasionally pointed this out (Garstang, 1922; deBeer, 1958), but their admonitions have fallen mostly on deaf ears. As recently as 1976, biologist William Ballard (who, according to Richard Elinson, coined the term “pharyngula” [Elinson, 1987]), lamented the fact that so much energy continues to be “diverted into the essentially fruitless 19th century activity of bending the facts of nature to support second-rate generalities.” Ballard concluded that it is “only by semantic tricks and subjective selection of evidence” that one can argue that the early stages of the various classes of vertebrates “are more alike than their adults.” (Ballard, 1976, p. 38)

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References

Ballard, W. W.: 1976. Problems of Gastrulation: Real and Verbal. BioScience, 26: 36-39.

Darwin, C.: 1859. On the Origin of Species, reprint. New York: Modern Library.

de Beer, G.: 1958. Embryos and Ancestors, 3d ed. Oxford: Clarendon Press.

Elinson, R. P.: 1987. Change in Developmental Patterns: Embryos of Amphibians with Large Eggs. In Development as an Evolutionary Process,ed. R. A. Raff and E. C. Raff, Vol. 8, pp. 1-21. New York: Alan R. Liss.

Garstang, W.: 1922. The Theory of Recapitulation: A Critical Re-statement of the Biogenetic Law. Journal of the Linnean Society (Zoology), 35:81-101.

Gilbert, S. F.: 1994. Developmental Biology, 4th ed. Sunderland, MA.: Sinauer Associates.

Gould, S. J.: 1977. Ontogeny and Phylogeny. Cambridge, MA.: Belknap Press. Hall, B. K.: 1992. Evolutionary Developmental Biology. London: Chapman & Hall.

Lehman, H. E.: 1987. Chordate Development, 3d ed. Winston-Salem, NC: Hunter Textbooks.

Raff, R. A.: 1996. The Shape of Life: Genes, Development, and the Evolution of Animal Form. Chicago: The University of Chicago Press.

Wolpert, L.: 1991. The Triumph of the Embryo. Oxford: Oxford University Press.

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